I think it would be safe to buy off eBay if it comes in the original packaging. I, however, would opt for using Microsoft Security Essentials instead though for two reason.

The first is that Microsoft has a vested interest in keeping your computer safe so you don’t switch over to a Mac because they are known to be safer.

The second is that it is free. I understanding, from a corporate management perspective, that Microsoft Security Essentials isn’t ideal because they don’t have the management tools as the big companies like McAfee but for home use Microsoft Security Essentials is perfect.